Protection of human beings from extremely hot environments is of significant practical importance. Oil field workers in a broiling summer desert, steel makers in front of furnaces, molders and forgers handling fiery work pieces, and fire fighters in a burning house all need protection from extremely hot environments. However, no efficient cooling method is available at the present time. Forced convective cooling has been proposed to cool the human body for persons working in an extremely hot environment, such as on-duty fire fighters. However, in this case, a cable is needed to connect the human body and the heat sink. This is impractical when the distance between the working position and the heat sink is relatively long. The cable is too heavy for a person to carry and may be tangled during operation, which could cause the coolant flow to cease in the cable. Also, the cable may restrain a worker's free movements that are needed for a fire fighter in a burning house or a petroleum drilling worker on a drilling platform.
Heat pipes combined with phase-change material (PCM) energy storage systems are a solution for human body temperature control. Heat pipes are heat transfer devices with a very high heat conductance. They can transfer a large quantity of heat from one place to another with a very small temperature drop. PCM energy storage systems absorb or release heat energy via melting at a nearly constant phase-change temperature. The incorporation of heat pipe/PCM energy storage systems in a human body temperature-control suit can effectively cool the human body at a fairly uniform and human-tolerable temperature.
The heat pipe is a device with an effective thermal conductance hundreds of times higher than that of copper. A tube or other type of container with a porous wick structure placed on the interior surface is filled with a small amount of working fluid. Air is evacuated from the container and the container is sealed. Heat is applied to the evaporator section, which causes the liquid to vaporize. The vapor then moves from the hotter section due to the higher vapor pressure to the colder section of the heat pipe, where it is condensed. The liquid condensate then returns to the heated section in the wick structure due to a capillary pumping action. Since the latent heat of vaporization is very large, high heat transfer rates can be achieved with a small temperature difference between the hot and cold sections, and consequently, the temperature is rather uniform along the heat pipe length. Gravity may be used to assist the liquid return and increase the heat transport capacity of the heat pipe. In this case, a capillary structure on the interior surface of the heat pipe may not be necessary. For more detailed descriptions on heat pipes, it is sufficient to cite a publication entitled, "Heat Pipe," by P. D. Dunn and D. A. Ready, Pergamon Press, Oxford, N.Y., 1982, where such a general description of heat pipes is contained in pages 1 to 20.
The heat pipe to be used in the human body cooling suit should be flexible without seriously restraining movement. The flexible heat pipe was first proposed to be used on human beings for hand protection from extreme cold by Faghri et al. in the paper entitled, "Heat Pipes for Hands," Mechanical Engineering, June, 1989, pages 70 to 74. They found that the human elbow has enough heat to keep the fingers warm in cold weather, and that the heat pipe proved to be an efficient means to deliver that energy.